Maritime Plant Hardiness Zones. Recently with global warming we have noticed that plants thought once could'nt grow here now can! Last year the news stated that more and more areas on PEI should be classified as zone 6a. The island has microclimates of zone 6b and colder ones of zone 4b (open and low swampy areas). A lot of the time you will be able to tell yourself by watching the leaves change in the fall where the coolest zones are and the warmest ones will be sheltered from the wind and the leaves will stay green longer. Now that is just common sense.